Cryptosporidium is now recognized as an important cause of diarrhea in healthy and immunocompromised hosts. It is frequently found in finished drinking water and is difficult to eradicate even when current water treatment standards are followed. Therapy of Cryptosporidiosis is sub-optimal and in certain populations (such as people with AIDS) it can result in significant morbidity. Cryptosporidium has been identified as a health threat by the Institute of Medicine and has been targeted for eradication from water and food by the Environmental Protection Agency and the Food and Drug Administration as a part of the Presidential safe food initiative. Cryptosporidium encompasses a heterogeneous group of species that traditionally have been classified by host preferences and more recently by genotype analysis. Cryptosporidium parvum is thought to be the only species that can infect humans; however, the infectivity of non-parvum species for humans has not been studied. C.parvum species has been divided into those infecting humans (genotype H) and those that are transmitted between human and animal hosts (genotype C). In previous studies, experimental cryptosporidiosis in healthy adults has revealed variability in infectivity, outcome, and immune response to geographically-diverse genotype C isolates. However, a significant proportion of naturally acquired cases of cryptosporidiosis are due to the H genotype. The successful passage of genotype H isolates in gnotobiotic piglets will allow us to extend volunteer studies to include genotype H isolates. Comparisons of infectious dose, clinical syndromes and immune response will be done. The relative virulence of these isolates will then be correlated in laboratory models of cryptosporidiosis. In addition, we will identify the mucosal cytokines that are associated with acute cryptosporidiosis (INJURY), resolution of oocyst excretion (CONTROL), and subsequent HEALING by examining intestinal biopsies from normal volunteers with and without symptoms at different time points after experimental infection with Cryptosporidium oocysts. We will confirm the role of Th1 cytokines in resolution of cryptosporidiosis by conducting a pilot, proof-of-concept, open-label trial of IL-12 as adjunctive therapy in chronic cryptosporidiosis in AIDS. In addition to identifying an agent of use in chronic AIDS associated cryptosporidiosis, the understanding of the outcomes of infection with the individual genotypes will allow the development of more accurate risk assessment models useful in the prevention of endemic cryptosporidiosis and large scale waterborne outbreaks due to Cryptosporidium.